Technical Field
The invention relates to the field of inkjet printing. More specifically the invention relates to systems and methods of applying a gaseous inhibitor into a printing region to hinder the curing process of ink on the print heads caused by the presence of stray light in the printing environment.
Description of the Related Art
Using electromagnetic radiation to cure liquid chemical formulations has been an established practice for many years. Electromagnetic radiation curing involves a liquid chemical formulation comprising photoinitiators, monomers and oligomers, and possibly pigments and other additives and exposing the formulation to electromagnetic radiation, thereby converting the liquid chemical formulation into a solid state.
In printing applications, radiation-curable ink is jetted from a print head onto a substrate to form a portion of an image. In some applications, the print head scans back and forth across a width of the substrate, while the substrate steps forward for progressive scan passes. In some other applications, one or more blocks of fixed print heads are used to build an image.
In each of these printing settings, curing ink involves directing photons, typically with wavelengths in or near the ultraviolet spectrum, onto an ink deposit. The photons interact with photoinitiators present within the ink, creating free radicals. The created free radicals initiate and propagate polymerization (cure) of the monomers and oligomers within the ink. This chain reaction results in the ink curing into a polymer solid.
However, the use of curable inks has created negative side effects. In particular, standard ink curing designs have issues with the print heads being exposed to stray light and with ink hardening onto the print heads due to the exposure. Stray light enters the printing environment in a variety of ways. For example, environmental light enters even the smallest openings and reflects throughout the system. Additionally, printing systems are oftentimes opened to environmental light to access printer components. Furthermore, printing systems sometimes produce their own light by way of scanner functions or curing lamps.
Exposure to any stray light encourages ink to harden onto print heads. The hardened ink subsequently deflects the spray from the print head and causes poor print quality. Indeed, even a very small deflection in ink spray can cause ruinous results.
In all types of printers which use light-curing (i.e. wideformat, super wide format, single pass, etc.), similar methodologies have been applied to limit the impact of stray or ambient light. Some workarounds include the use of physical shutters and baffles to deflect the light coming from the lamps. However, no matter how much shielding is used, stray light still enters the printer. Another attempted solution involves configuring a curing lamp at such an angle that the light cannot deflect back at the print-heads. However, this technique detracts from the lamp's effectiveness in curing. Another attempted approach involved configuring a shield around the print zone that stops ambient light, especially UV, from entering the printer and reaching the heads. However, as explained above, stray light still enters the printer.
A number of other factors exacerbate the problems associated with stray light. Firstly, there are issues with inks curing on heads where the substrates being printed are very reflective, such as metallic finish substrates and even glossy white substrates. In these cases the amount of reflected light is much higher than usual. Secondly, with the increase in cure speed of the printers, both the ink sensitivity to UV light and the amount of light applied have increased substantially, thereby causing increased risk of ink curing on the heads. Thirdly, there are instances in printer design, where there is insufficient room to effectively shield the heads from stray light from the source.
Moreover, light emitting diodes (LEDs) are now predominately used for ink curing. The LEDS used operate at wavelengths in the upper band of the visible spectrum and into the ultraviolet spectrum and the ink is designed to be cured at these wavelengths. Accordingly, environmental light is particularly troublesome since environmental light contains a lot of energy in that band.
Yet another complication to the problem of stray light arises from the practice of using gaseous nitrogen in a print system to supplant oxygen. The presence of oxygen at the ink surface inhibits the curing reaction from occurring within the ink. This is often referred to as oxygen inhibition. Accordingly, the practice of supplanting oxygen in a curing region increases the efficiency of the cure process. However, nitrogen curing results in escaped nitrogen exposed to the print region, thereby exacerbating the problem of ink becoming cured to the printer heads.